Compression Check
 

I am going to do a compression check on my starboard Merc 400 (454). What kind of reading should I expect? 110- 130 lbs?

Wannabe

That's low for an engine with not much cam. Leakdown is a much more definitive test.

In very good shape, 140lbs with less than a 10% variance from the highest to lowest. An engine with some hours will most likely be lower, like around 120lbs. The key thing to look for is a large variance and that will indicate a problem.

Remember- warm engine, throttle wide open.

This will be a cold check. Starboard engine backfiring 1300-1800 rpm and maxes out at 4500 rpm. Port engine no backfire and 5000 rpm. Suspect a faulty carb (holley) but will check # 4 cylinder- previous owner gave me heads up on this.

Wannabe

Take the compression tester and remove the tire valve from it. Then screw it into the hole you want to check and pressurize it. You'll have to hold the crank- a breaker bar on the balancer bolt will work. Listen in the intake , exhaust and valve cover breather. Poor man's leakdown test. If it's backfiring you may have a burnt exhaust valve.

It would be easier to swap the carbs than to do a compression test.

Chris- Thought about the switching carbs thing. Concern is If it is a valve I will mess that carb up to.

Wannabe

Yeah, you could pop a power valve.

Look at the bright side- it's February. This isn't a problem you want to find on July 1st.

I had an Thunderbolt IV ignition module go out which didn't allow the timing to advance. I don't know about your boat, but on mine it was quick to check the advance at ~3,000rpm.

Also, one summer I had an alternator dying on me. It wasn't completely dead but dead enough to let the voltage drop causing the engine to pop occasionally. After a long day on the water I noticed the popping got worse when I hit the trim tabs - that's when I put a volt meter on it and found the problem. The reason it didn't show up earlier was I'm very religious about charging the batteries before every trip.

Just a couple of ideas you can rule out with quick simple checks. Not likely the problem, but can't hurt to rule them out.

Is it a steady pop? If its a steady backfiring reapeatedly, it could be a cam lobe went flat, had that happen.

Mine backfired repeatedly thru the carb as soon as I cracked the throttle open.

First: Don't buy Champion Plugs
 

We pulled the starboard plugs first and had 2 big OH-NO's. # 7 cylinder central electode cracked and missing as well as part of pocelain insulator. Same on # 2 cylinder. NO COMPRESSION on # 7. POS Champion plugs can't last 40 hours. Smell of gas in oil. several plugs were loose.

Port engine- Plugs all look normal- all tight.

We gapped 16 new AC MR43T to 0.40" and installed.

I have to call Tyler Crockett to talk about fixing the bum engine.

Wannabe

If the plugs came loose, that could be the reason the electrodes cracked, and could damage a valve.

The gas in oil, could be from running with the bad plugs, not burning the gas, and washing down into the crankcase.

Might just have to replace the valve(s) in the #7 cylinder, and install new plugs tight, and could be ok.

I know myself, I have installed the plugs all finger tight, and forgot to torque them down, resulting in loose plugs and cracked electrodes.

I hope it's the valves and not a cracked piston. I agree the gas smell is from the cylinder being washed down.

The original backfiring was through the intake- not the exhaust.

It's like 2 different people rebuilt the 2 engines. Dufus did the starboard engine and Dinkus did the Port engine.

Either way the Champion plugs stink. They were the Non- marine kind.

Wannabe

Throw all the Champion spark plugs into the water and only use the ones that float.

I agree !!!!

Wannabe

Detonation will sometimes cause the plugs to loosen. It will also cause the insulators to break. I'd check that out carefully

Patriyacht- I am aware of the possible detonation issue- thanks.

The engines were over advanced a couple of degrees. The plugs on the Port engine look good.

Wannabe

Keeping my fingers crossed it's only a bad valve. Good luck.